The present invention relates to a load lock unit for transferring a cassette of semiconductor wafers between regions having different pressures or environments and a method for transferring the wafer cassette through the load lock unit. More particularly, the invention relates to a dual stage wafer load lock unit wherein first and second chambers of the load lock are selectively separated by a platform carrying the wafer cassette, and a method of transferring wafers in a cassette through the dual stage load lock.
Semiconductor wafer processing typically occurs within high vacuum process chambers which are separated from adjacent chambers and/or external conditions (ambient or atmospheric pressure) by valved openings. Typically, a central transfer chamber is provided with a transfer robot located therein for transferring wafers between a load lock chamber and one or more process chambers located about the periphery of the transfer chamber. The load lock chamber is also provided for transferring wafers between the transfer chamber, which is typically at high vacuum, and the outside, typically a clean room at atmospheric pressure.
One well known prior art system for loading wafers into a process system employs a wafer cassette, having a plurality of individual slots therein, within which individual wafers may be placed or removed. This cassette is placed onto a moveable floor or platform in the load lock chamber, and an elevator mechanism moves the floor in increments to sequentially place, or index, individual slots of the cassette into registration with a robot blade which is used to selectively remove wafers from the cassette or place wafers therein. A slit valve, located in alignment with the robot blade, isolates the load lock chamber from the robot/transfer chamber. To enable access of the robot to all of the slots in the cassette, the cassette must be moveable, within the load lock, to align the wafer slots at each end of the cassette with the robot blade. As a result, the volume of the load lock chamber must be approximately at least twice the volume of the cassette, to create the clearance needed to move the cassette the entire travel length required to access each wafer slot in the cassette with the robot while maintaining the cassette within the controlled environment of the load lock.
To avoid a backflow of ambient air into the transfer chamber, when the valve between the two chambers is opened to transfer a wafer, it is essential that the load lock chamber be maintainable at a pressure substantially equal to the high vacuum of the transfer chamber. Thus, because of the need to have a load lock volume significantly exceeding that of the cassette to enable cassette movement, and thereby robot blade access to each wafer slot in the cassette, a large dead space equal to the required cassette clearance space must be pumped down to vacuum each time a cassette is replaced in the load lock or the load lock door is opened. Additionally, exposing the interior surfaces of the load lock chamber to atmosphere by opening of the load lock door for cassette loading or unloading allows water vapor and contaminants to enter the load lock chamber and adhere thereto.
Accordingly, there is a need in the art for a load lock chamber which provides cassette to wafer indexing while minimizing the volume and space of the load lock which is exposed to ambient conditions.